1. Field of the Invention
The present invention relates to an initial ball forming method for a wire used in wire bonding and to a wire bonding apparatus.
2. Prior Art
In wire bonding techniques, input and output terminals, etc. (bonding pads) of semiconductor chips such as LSIs, etc. and terminals (bonding leads) of packages or circuit boards on which the semiconductor chips are mounted are connected by fine metal wires.
Gold wires, for instance, are used as the wires that connect bonding pads and bonding leads; and when such gold wires are initially bonded to the bonding pads, the tip end of a gold wire that is passed through a tube called a capillary is formed into a ball shape. The ball-shaped entity that is formed on the tip end of this gold wire is generally called an initial ball. Such a general initial ball is obtained by applying a high voltage across the wire and a torch electrode that is disposed so as to face the tip end of the wire, so that a space discharge is caused to take place between the electrode and the wire, thus melting the tip end of the wire by the energy of this discharge, thus making the tip end into a ball shape. In this case, the material of the torch electrode that is used generally has a higher melting point than that of the wire material.
The process that connects by wires the bonding pads and bonding leads is performed in two stages. First, a first-stage bonding operation is performed by pressing the general initial ball against the aluminum bonding pad of the semiconductor chip, and applying ultrasonic energy while heating this initial ball. Then, the wire is extended from the end that is bonded to the aluminum bonding pad, and the other end of the wire is moved to a point above the gold bonding lead of the circuit board to which the wire is to be bonded. Then, a second-stage bonding operation is performed by pressing the other end of the moved wire against this bonding lead, and applying ultrasonic energy while heating the wire.
In the bonding of the bonding pad and gold wire, a specified wire bonding strength, e.g., peeling strength, can be obtained by setting the pressing pressure, heating temperature and ultrasonic energy that is applied at appropriate values. The thermal energy created by heating has the function of making it possible to reduce the ultrasonic energy that is required; for example, in cases where heating can be performed at a sufficiently high temperature, bonding can be accomplished without applying ultrasonic energy. There are various theories regarding the mechanism of the bonding between an aluminum bonding pad and a gold wire; however, it appears that the aluminum surface oxide film is ruptured by the energy that is applied, and that the underlying newly generated aluminum surface and the gold are bonded by a type of eutectic phenomenon.
Thus, in the case of conventional wire bonding, gold wires have generally been bonded to aluminum bonding pads. In recent years, as ultra-fine miniaturization of LSIs has progressed, copper wiring techniques which makes it possible to lower the wiring resistance inside semiconductor chips have attracted attention. In such cases, it is preferable to use copper materials for the bonding pads, both from the standpoint of the process and the standpoint of materials.
However, bonding of gold wires to copper bonding pads involves several problems. More specifically, a copper oxide film is formed on the surfaces of copper bonding pads, and this oxide film on the copper surface has extremely poor bonding characteristics compared to the oxide film formed on the surfaces of aluminum bonding pads. For example, even if the general initial ball of a gold wire is pressed against the surface of a copper bonding pad and ultrasonic energy of considerable strength is applied, this merely results in the generation of several fractures in the general initial ball; and no bonding occurs between the copper and gold. Furthermore, when the copper surface is heated, oxidation occurs more violently as the metal color of the surface varies conspicuously. Accordingly, it is difficult to utilize thermal energy generated by heating.
This problem comes from the fact that the manner in which copper is oxidized differs from the manner in which aluminum is oxidized. In other words, as is well known, the surface of aluminum is easily oxidized; however, the resulting oxide film is dense; and when this film grows to a certain fixed film thickness, there is almost no further oxidation. Furthermore, the surface oxide film on aluminum is also stable in chemical terms, and any oxide film of this type has more or less fixed physical characteristics. Accordingly, the wire bonding conditions such as pressing pressure, heating temperature, ultrasonic energy, etc. that are used to rupture the surface oxide film on an aluminum bonding pad can be set with good reproducibility.
On the other hand, the oxide film that is formed on the surface of copper has a low density, and oxidation continues to progress toward the interior of the copper. Furthermore, in the case of a copper oxide film, both the film thickness and the physical properties show a large amount of variation according to the degree of oxidation. Accordingly, a thick copper oxide film has great variation in physical properties, etc., which cannot be compared with the surface oxide film on an aluminum bonding pad, is present on the surface of a copper bonding pad. Furthermore, the presence of this surface oxide film on a copper bonding pad constitutes a great impediment to wire bonding.